A major portion of the worldwide petrochemical industry is concerned with the production of light olefin materials and there subsequent use in the production of numerous important chemical products via polymerization, oligomerization, alkylation and the like well-known chemical reactions. Light olefins include ethylene, propylene and mixtures thereof. These light olefins are essential building blocks for the modern petrochemical and chemical industries. The major source for these materials in present day refining is the steam cracking of petroleum feeds. For various reasons including geographical, economic, political and diminished supply considerations the art has long sought a source other than petroleum for the massive quantities of raw materials that are needed to supply the demand for these light olefin materials. In other words, the holy grail of the R & D personnel assigned to work in this area is to find a way to effectively and selectively use alternative feedstocks for this light olefin production application thereby lessening dependence of the petrochemical industry on petroleum feedstocks. A great deal of the prior art's attention has been focused on the possibility of using hydrocarbon oxygenates and more specifically methanol as a prime source of the necessary alternative feedstock. Oxygenates are particularly attractive because they can be produced from such widely available materials as coal, natural gas, recycled plastics, various carbon waste streams from industry and various products and by-products from the agricultural industry. The art of making methanol from these types of raw materials is well established and typically involves the use of one or more of the following procedures: (1) manufacture of synthesis gas by any of the known techniques typically using a nickel or cobalt catalyst followed by the well-known methanol synthesis step using relatively high pressure with a copper-based catalyst; (2) selective fermentation of various organic agricultural products and by-products in order to produce oxygenates; or (3) various combinations of these techniques.
Given the established and well-known technologies for producing oxygenates from alternative non-petroleum raw materials, the art has focused on different procedures for catalytically converting oxygenates such as methanol into the desired light olefin products. These light olefin products that are produced from non-petroleum based raw materials must of course be available in quantities and purities such that they are interchangeable in downstream processing with the materials that are presently produced using petroleum sources. Although many oxygenates have been discussed in the prior art, the principal focus of the two major routes to produce these desired light olefins has been on methanol conversion technology primarily because of the availability of commercially proven methanol synthesis technology. A review of the prior art has revealed essentially two major techniques that are discussed for conversion of methanol to light olefins. The first of these MTO processes is based on early German and American work with a catalytically conversion zone containing a zeolitic type of catalyst system. Representative of the early German work is U.S. Pat. No. 4,387,263 which was filed in May of 1982 in the U.S. without a claim for German priority. This '263 patent reports on a series of experiments with methanol conversion techniques using a ZSM-5-type of catalyst system wherein the problem of DME recycle is a major focus of the technology disclosed. Although good yields of ethylene and propylene were reported in this '263 patent, they unfortunately were accompanied by substantial formation of higher aliphatic and aromatic hydrocarbons which the patentees speculated might be useful as an engine fuel and specifically as a gasoline-type of material. In order to limit the amount of this heavier material that is produced, the patentees of the '263 patent propose to limit conversion to less than 80% of the methanol charged to the MTO conversion step. This operation at lower conversion levels necessitated a critical assessment of means for recovering and recycling not only unreacted methanol but also substantial amounts of a DME intermediate product. The focus then of the '263 patent invention was therefore on a DME and methanol scrubbing step utilizing a water solvent in order to efficiently and effectively recapture the light olefin value of the unreacted methanol and of the intermediate reactant DME.
This early MTO work with a zeolitic catalyst system was then followed up by the Mobil Oil Company who also investigated the use of a zeolitic catalyst system like ZSM-5 for purposes of making light olefins. U.S. Pat. No. 4,587,373 is representative of Mobil's early work and it acknowledged and distinguished the German contribution to this zeolitic catalyst based MTO route to light olefins. The inventor of the '373 patent made two significant contributions to this zeolitic MTO route the first of which involved recognition that a commercial plant would have to operate at pressure substantially above the preferred range that the German workers in this field had suggested in order to make the commercial equipment of reasonable size when commercial mass flow rates are desired. The '373 patent recognized that as you move to higher pressure for the zeolitic MTO route in order to control the size of the equipment needed for commercial plant there is a substantial additional loss of DME that was not considered in the German work. This additional loss is caused by dissolution of substantial quantities of DME in the heavy hydrocarbon oil by-product recovered from the liquid hydrocarbon stream withdrawn from the primary separator. The other significant contribution of the '373 patent is manifest from inspection of the flow scheme presented in FIG. 2 which prominently features a portion of the methanol feed being diverted to the DME absorption zone in order to take advantage of the fact that there exist a high affinity between methanol and DME thereby downsizing the size of the scrubbing zone required relative to the scrubbing zone utilizing plain water that was suggested by the earlier German work.
Primarily because of an inability of this zeolitic MTO route to control the amounts of undesired C4+ hydrocarbon products produced by the ZSM-5 type of catalyst system, the art soon developed a second MTO conversion technology based on the use of a non-zeolitic molecular sieve catalytic material. This branch of the MTO art is perhaps best illustrated by reference to UOP's extensive work in this area as reported in numerous patents of which U.S. Pat. No. 5,095,163, U.S. Pat. No. 5,126,308 and U.S. Pat. No. 5,191,141 are representative. This second approach to MTO conversion technology was primarily based on using a catalyst system comprising a silicoaluminophosphate molecular sieve (SAPO) with a strong preference for a SAPO species that is known as SAPO-34. This SAPO-34 material was found to have a very high selectivity for light olefins with a methanol feedstock and consequently very low selectivities for the undesired corresponding light paraffins and the heavier materials. This SAPO catalyzed MTO approach is known to have at least the following advantages relative to the zeolitic catalyst route to light olefins: (1) greater yields of light olefins at equal quantities of methanol converted; (2) capability of direct recovery of polymer grade ethylene and propylene without the necessity of the use of extraordinary physical separation steps to separate ethylene and propylene from their corresponding paraffin analogs; (3) sharply limited production of by-products such as stabilized gasoline; (4) flexibility to adjust the product ethylene-to-propylene weight ratios over the range of 1.5:1 to 0.75:1 by minimal adjustment of the MTO conversion conditions; and (5) significantly less coke make in the MTO conversion zone relative to that is experienced with the zeolitic catalyst system.
Despite the promising developments associated with the SAPO catalyzed MTO route to light olefins, the problem of DME co-production is common to both types of catalytic MTO routes discussed above and various measures have been suggested in the prior art to recover and recycle DME from the effluent stream from an MTO conversion zone. In U.S. Pat. No. 4,387,263, a relatively high pressure DME absorption zone is taught utilizing a plain water solvent in order to recapture and recycle the DME intermediate. By the use of the term “high pressure” with reference to this '263 patent, it is pointed out that the examples 1, 2, 3 and 4 were run at 2000 kPa (290 psi) and the fifth example was run at an even higher pressure of 4000 kPa (580 psi). One of the improvements suggested by U.S. Pat. No. 4,587,373 focused on utilizing a more efficient DME solvent in the DME absorption zone and recommended that a portion of the methanolic feed to the MTO conversion reactor be diverted to the DME absorption zone in order to more efficiently recapture the DME contaminant from the olefin product stream. As explained above, this '373 patent proposed to reduce commercial plant size by operating the MTO conversion reactor at a much higher preferred pressure than was suggested by the prior art and specifically focused on a reactor operation at about 550 kpa (80 psi) but noted that operating the MTO reactor at this high pressure would open the door to substantial DME loss in the heavy hydrocarbon product stream recovered from the principal separator in the effluent work-up portion of the flow scheme unless steps were taken to strip dissolved DME from the heavy hydrocarbon by-product stream. In particular, in the flow scheme of FIG. 2 of the '373 patent, it is proposed to strip the heavy hydrocarbon by-product recovered from the primary separator 16 in stabilizer tower 26 in order to recapture the value of the DME intermediate dissolved therein while simultaneously using a methanol solvent in the DME absorber 22.
In my attempts to practice a product recovery flow scheme quite similar to that disclosed in FIG. 2 of the '373 patent in conjunction with the use of a SAPO-type catalytic system in a MTO conversion zone, I have now found a further problem associated with the use of this flow scheme in order to recapture and recycle the DME intermediate that contaminants the effluent stream from the MTO reaction zone. I have found more specifically that if a portion of the methanol feed to the MTO conversion zone is diverted to the DME absorber as suggested in the '373 patent in order to recover DME more efficiently, there is substantial co-absorption of light olefins into the methanol solvent associated with this scheme. A methanol solvent's ability to extract from the light olefin-containing input stream to the DME absorber not only DME but substantial quantities of C2 and C3 olefins was not reported in the '373 patent and greatly complicates the design of an efficient product work-up flow scheme for a SAPO based MTO conversion zone. For example, when the DME absorption zone is operated with a methanol solvent at scrubbing conditions including a temperature of about 54° C. (129° F.) and a pressure of about 2020 kPa (293 psi) with a 99.85 mass-% methanol solvent, at least 12.3 mass-% of the C2 olefins and 40.3 mass-% of the C3 olefins charged to the DME scrubber will be co-absorbed in the DME-rich liquid solvent bottom stream withdrawn from the scrubber. When this DME-rich solvent stream is recycled to the MTO conversion zone, a substantial internal circuit of light olefins is created which acts to substantially increase the size of the MTO conversion zone while increasing the rate of detrimental coking on the catalyst contained therein due to the fact that these C2 and C3 olefins are reactive and can undergo polymerization and condensation to form coke precursors.
The problem addressed by the present invention is therefore to substantially diminish this undesired buildup of C2 and C3 olefins in the DME recycle stream flowing to the MTO conversion zone when methanol is used as a solvent in a DME absorption zone that is a prominent feature of an MTO reactor effluent work-up scheme.